Surface Tension

Leo Anchorage, Alaska

Leo Anchorage, Alaska

Many things in the environment are related to the tendency of water to stick to itself (cohesion) and the tendency of water to stick to other things (adhesion). Surface tension and contact angle are measures of cohesion and adhesion. Rain comes in drops, generally of a small range in size because of surface tension. Small droplets (mist) in the atmosphere coalesce into larger rain drops because of the tendency of water to stick to itself. As the droplets increase in size the larger mass requires more energy to hold it together causing larger drops to break apart. The counteracting effects of surface tension and larger size interact to provide us with rain drops of the sizes seen.

Contact angle is the angle a drop of water makes with a surface. The contact angle is a visual representation of the tendency of the droplet to be be attracted to or repelled from the material. If the droplet is attracted to the surface or textile then it will eventually wet up. If the droplet is repelled the water will eventually roll off. More technically the degree of repulsion or attraction is proportional to the cosine of the contact angle. A contact angle of zero degrees gives a cosine equal to 1, and the surface is rapidly wet with the water. A contact angle of 180 degrees gives a cosine of -1 and the water is fully repelled. The cosine of the contact angle is zero at 90 degrees (a right angle) and switches from positive (water attracted to surface) to negative (water repelled) at 90 degrees.

The contact angle for most rocks and soils is zero meaning the rain fully wets the surface as it falls. Materials that wet quickly such as cotton are called hydrophilic (water loving). The water is attracted to the surface of rock and soils, they are hydrophilic. Materials such as wax are hydrophobic (water hating). Water beads up on hydrophobic materials. Hydrophilic materials have low contact angles (i.e., the water bead spreads into a film). Hydrophobic materials have a high contact angle so the water sticks to itself, forming a beads (small separate droplets).

Contact Angle on Water Repellent Fabric

Figure 1. Contact angle on a water repellent fabric.

Surface tension and contact angle are critical determinants in how water moves through partially saturated soil. Examine any water surface such as a pool, stream, or lake. Notice how the soil is wet, sometimes even saturated, some distance above the water level. This is called the capillary fringe. Water moves upward through the soil from the water surface into the capillary fringe. The same process also occurs in the soil moving away from the water surface, but in this case it is not visible. In fact it occurs on land everywhere beneath our feet. The height of the capillary fringe above the water level varies. In fine grained soils (clay and silt) the capillary fringe reaches high above the water surface, sometimes a meter or more. In coarse grained soils the capillary fringe is much smaller. On rocks and gravel there is no capillary fringe. Again we have a scale effect caused by the interaction of two opposing forces. The attraction of the water for the soil causes it to be pulled into the soil or rise in small openings (capillaries) between soil grains. However as the water rises further above the surface the weight of the water held in the capillaries increases. Smaller pores present in finer grained soils have a greater fraction of mineral surface to attract the water in comparison with the weight of the water than is the case with coarse materials such as gravel. The physics dictate that water rises to a greater height in finer materials than coarse.

What physics? Assume for simplicity that the convoluted pores in the soil are a cylindrical tube. The upward force (attraction between soil grains and water) is proportional to the surface area of the capillary times the height of the water column (2 radius height). The downward force (the weight of the water) is proportional to the volume of water held in the capillary ( radius radius height). The weight of water depends upon the radius squared whereas the upward force depends upon the radius. As the radius of the capillary (soil grain size) increases the weight of the water overcomes the surface tension and capillary rise is small.

Capillary forces are important as wicks in candles and some stoves, and in moving liquid water through clothing. The importance of surface tension in clothing performance is illustrated in Figure 2. A waterproof breathable membrane is illustrated, however this could be most any waterproof/breathable fabric. One side of the fabric, the warm side, has a human body inside generating heat. Water vapor moves in response to changes in vapor pressure or concentration of water in the air. For illustration assume that liquid water is present as sweat on the skin and outside the fabric as raindrops. The concentration of water vapor in the air in equilibrium with liquid water depends primarily on temperature. The warmth of the human body inside the jacket increases the vapor pressure inside the jacket relative to outside the jacket causing the water vapor to diffuse through the pores in the membrane into the outside air and even into the rain drops resting on the outside surface of the membrane.

Membranes of this type can continue to transfer water based upon the change in temperature even with liquid water running against both sides of the membrane. One type of water desalination, direct contact membrane distillation, works in exactly this manner.

how waterproof/breathable membranes work

Figure 2. How waterproof/breathable membranes work.

Notice the rain drops drawn on the outside of the membrane. What keeps the water droplets from passing through the pores? The answer is surface tension. The membrane material is hydrophobic so the water sticks to itself, not the membrane. In order for the water droplet to pass through the pore, sufficient pressure must be applied to overcome the surface tension. This is possible if the water is placed under pressure or if something reduces the surface tension. Interestingly the pore size of most waterproof/breathable fabrics is the same as that in most portable water filters used for water purification. Only surface tension and the hydrophobic nature of the fabric keep the liquid water out.
Given enough pressure, or wetting out, water will pass through the pores; only surface tension keeps the water out. Soaps, smoke, alcohol, and other contaminants may reduce the surface tension of water, causing the membrane to leak.

The surface tension of water is a property of the boundary between air and water. Any chemical which tends to accumulate preferentially in this boundary layer can tend to reduce surface tension. Such chemicals are called surfactants. Soaps and detergents are examples of surfactants. Surfactants usually consist of relatively large molecules which are polar and tend to orient themselves to exclude other molecules at the water/air surface. Cleaning of a fabric is accomplished when the fabric becomes wet with water rather than oils. Surfactants assist with this wetting and thus promote cleaning. Surfactants also promote the wetting of many waterproof/breathable materials, leading to leakage. The contact angle of water with the fabric is determined by the interfacial tension between all three phases (water, air, fabric). As the surface tension of the water declines, the contact angle declines, leading to wetting. Besides surfactants alcohol as used in stove fuels reduces the surface tension of water and can make waterproof breathable materials leak.

Wetting of a fabric can change with conditions. Many times materials may be hydrophobic and water beads on the surface. After continuous exposure portions of the material may wet out. Wetting out means that water contacts and is bound to the fabric fibers, rather than air. The wetting out will occur more rapidly if the material is dirty or contaminated with a surfactant (e.g., soap or detergent). At this point the barrier to water transport through the pores in the fabric is removed. Membrane materials are also subject to wetting out at some locations. When membranes used in clothing wet out they are capable of transmitting liquid water (leaking). The hydrophobic properties are generally returned if the material is allowed to thoroughly dry and cleaned to remove any surface active agents. Ironically, most modern rain gear works best when clean and dry. Don’t pack your breathable rain gear in a stuff sack or wadded in a tight wad when wet if at all possible because some of the pores may wet out. Keep the gear as dry as possible so that it remains waterproof. This is also true of soft shell garments.